1. Field of the Invention
The present invention generally relates to bearing device for supporting a motor shaft. More particularly, the present invention pertains to a bearing device for use with a motor shaft having a worm gear on its rotor shaft.
2. Description of the Related Art
A conventional motor shaft includes a worm-like thread that meshes in a worm wheel, so that rotational motion can be transferred between shafts that are disposed perpendicularly to each other. The output rotational speed is significantly reduced, and is a function of the gear ratio of the thread with respect to the worm wheel. This configuration is used in many applications where speed reduction is required, such as in power window motors, motors for reclining seats and wiper motors.
As illustrated in FIG. 6, a rotor shaft 80 of the motor includes a distal end and a proximal end. The distal end is supported by a radial bearing 84 for preventing the rotor shaft 80, from shifting in the radial direction.
The rotational motion of the rotor shaft 80 is transferred to the worm wheel by the thread 86 of the worm gear. The counter force generated by the worm wheel affects the rotor shaft 80. The rotor shaft 80, whose ends are supported by the radial bearings 82 and 84, is caused to bend in the direction opposite to the worm wheel side, under the effect of the counter force. Therefore, the magnitude of the gear engagement between the thread 86 and the worm wheel becomes smaller. The rotational stress caused by the bend in the shaft 86, reduces its strength. The size of the rotor shaft 80 and the radius of the gear teeth would normally be increased in order to reduce its rotational stress. Consequently, the reduction in the size and weight of the motor is difficult to achieve.
The Japanese Unexamined Utility Model Publication No. 60-103743 discloses an improved bearing device for a motor shaft, which addresses the foregoing drawbacks. In this bearing device, the rotor shaft 80 is supported by three radial bearings. As illustrated in FIG. 7, a first and second radial bearings 82 and 84 are provided at the distal and proximal ends of the rotor shaft 80, respectively. A third radial bearing 90 is also disposed at the central section of the rotor shaft 80, between the thread 86 and a rotor 88. When the rotor shaft 80 is supported at three points, the magnitude of the bend of the rotor shaft 80 is compensated by the third radial bearing 90. Therefore, the size and weight reduction in the motor can be achieved.
However, when the rotor shaft 80 is supported by three points, the axial centers of the three radial bearings 82, 84 and 90 should be aligned. An offset in the alignment of the three radial bearings 82, 84 and 90 or a dimensional error generated in the various assembled parts of the motor, generates friction that will impede the smooth rotation of the rotor shaft 80, such that the rotational speed of the rotor shaft is undesirably affected. Generally, if the rotational speed fluctuates, the current induced in a coil also fluctuates. Therefore, the electric drive circuit is undesirably affected by this current fluctuation, and it becomes difficult to maintain the motor performance. Furthermore, the offset in the alignment of the three radial bearings 82, 84 and 90 generates undesirable vibration, such that the rotation of the rotor shaft 80 becomes unstable.
Even if the rotor shaft 80 were supported at three points, the flexible bent of the rotor shaft 80 is not completely eliminated. As illustrated in FIG. 8, the distal end of the rotor shaft 80 contacts the inner peripheral surfaces of the associated radial bearings and generates noise. Noise is also similarly generated even when the rotor shaft 80 is supported at two points.
If the rotor shaft were bent at the vicinity of the thread, the motor would not smoothly start, since the radial bearing 82 does not support the rotor shaft 80 evenly.